Electrophoretic coating of aluminum and tin articles

ABSTRACT

In the process of coating a film-forming polymer on aluminum or tin article by electrophoretic process, the improvement which comprises coating the aluminum or tin article, prior to the electrophoretic process, with a layer of zinc, copper or zinc and copper.

United States Patent [1 1- Chu 1 Oct. 30, 1973 ELECTROII-IORETIC COATING OF ALUMINUM AND TIN ARTICLES [75] Inventor: Edward Chu, Philadelphia, Pa.

[7 3] Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: May 11, 1971 [21] Appl. No.: 142,384

[52] U.S. Cl. 204/181 7/1970 Hartzell et al. 204/ 181 Primary Examiner-Howard S. Williams Attorney-David J. Gould [57] 7 ABSTRACT In the process of coating a film-forming polymer on aluminum or tin article by electrophorctic process, the improvement which comprises coating the aluminum or tin article, prior to the electrophoretic process, with a layer of zinc, copper or zinc and copper.

14 Claims, No Drawings ELECTROPHORETIC COATING OF ALUMINUM AND TIN ARTICLES BACKGROUND OF THE INVENTION This invention relates to an improvement in the process of coating a film-forming polymer on an aluminum or tin article by electrophoretic process.

When aluminum or tin articles are coated with a filmforming polymer by electrophoretic process, the films have been found to be less than entirely satisfactory because they often are porous and blistered. It is believed that this is due to gas that is generated at the surface of the aluminum or tin article during the electrophoretic process. Unsatisfactory films are often especially apparent when a relatively high molecular weight polymer (having a molecular weight greater than about 10,000) is coated onto the aluminum or tin article.

SUMMARY OF THE INVENTION According to the present invention there is provided:

In the process for coating a film-forming polymer on an aluminum or tin article by electrophoretic process the improvement comprising coating said article prior to said electrophoretic process with a layer of zinc, copper or zinc and copper, the layer being continuous andof a sufficient thickness to preclude substantially any gas bubbles from being formed at the aluminum or tin article when said article is coated with the filmforming polymer by the electrophoretic process.

It has been found that the process of this invention provides excellent films on aluminum or tin articles; such films are not porous or blistered.

DESCRIPTION OF THE INVENTION Any suitable method can be used to coat the aluminum article with zinc or copper. These methods include providing such a layer of electrodeposition, immersion in a bath containing salts of copper or zinc, dipping in a molten bath of copper or zinc, and extrusion whereby a layer of copper or zinc is drawn over the aluminum article. Suitable methods are described in Surface Treatment of Aluminum by S. Wernick and R. Pinner, Chapters 13-16, third edition (1964), published by Robert Draper Ltd., Great Britain, the disclosure of which is hereby incorporated by reference.

Any suitable method can be used to coat the tin article with zinc or copper. These methods include providing such a layer by electrodeposition, dipping in a molten bath of zinc or copper and extrusion whereby a layer of copper or zinc is drawn over the tin article.

A preferred method of providing a layer of zinc on the aluminum article is the zinc immersion process. In this process the aluminum article, preferably after it has been cleaned, is immersed in a solution containing zinc ion.

In a preferred zinc immersion process, a zincate solu tion is used which is normally made up of zinc oxide and caustic such as sodium hydroxide. Generally, the higher the caustic concentration, the lower the weight of zinc will be deposited. Often zincate solutions will contain about 40-50 grams ofzinc oxide and about 400-450 grams of sodium hydroxide per each liter of water. Other additives can be contained in the zincate solution such as ferric chloride, Rochelle salt, sodium nitrate and the like. Often the zincate process will be carried out at 6080F. for about seconds to about 5 minutes.

Another zinc immersion process uses a solution of zinc fluoroborate, nickel fluoroborate and ammonium chloride. A suitable composition contains about grams zinc fluoroborate, 0.05 grams nickel fluoroborate, and 20 grams ammonium chloride per each liter of water.

Another zinc immersion process is the use of zinc sulfate and hydrofluoric acid. A suitable composition contains about 719 grams zinc sulfate and 35 milliliters of hydrofluoric acid per each liter of water.

A suitable method for the electrodeposition of zinc on the aluminum article is the Vogt process. Often, this process uses a bath containing about 0.5 gram zinc chloride, 0.5 gram sodium cyanide and 10.0 grams caustic soda per each liter of water. Ordinarily, the bath is operated at room temperature and at a low current density of about 5 amps per square foot of aluminum article. The time of plating is normally about 20 seconds. Prior to the electrodeposition, the aluminum article should be thoroughly cleaned.

Another suitable method for the electrodeposition of zinc on aluminum is Ore s variation of the Vogt process using a bath containing about 2.5 grams zinc chloride, 1.25 grams potassium cyanide, 25 grams caustic soda and 7 cubic centimeters of ammonia per each liter of water. The conditions of operation are room tempera-- ture, current of about 3 volts and plating time of about 3-4 seconds. Prior to electrodeposition, the aluminum article should be thoroughly cleaned.

Another suitable method for electrodeposition of a layer of zinc on the aluminum article is described in U.S. Pat. application Ser. No. 101,145 filed Dec. 23, 1970 in the name of John D. Rushmere, the disclosure of which is hereby incorporated by reference. The following is a summary of the Rushmere process:

In an aqueous alkaline zincate electroplating bath essentially free of cyanide, the. improvement which comprises having present in the bath an amount of bathsoluble N-substituted polyethyleneimine sufficient to produce an adherent zinc electroplate, substituted polyethyleneimine having an N-substituent group of the formula (CH ),,X wherein n is a positive integer of 0 to 4 inclusive, and

X is hydrogen, hydroxy, carboxy, carboxyalkylester of one to four carbon atoms in the alkyl group, carboxyamide, glycidyl, cyano, alkyl ether of one to four carbon atoms in the alkyl group, halogen and carbonyl.

In the Rushmere process, preferably the N- substituted polyethyleneimine is present in an amount of about 0.1 to 10 grams per liter and has a molecular weight of about 600 to 60,000. A preferred N- substituted polyethyleneimine is polyethyleneimine.

The above described methods for electrodeposition can be used also to provide a layer of zinc on a tin article.

Another suitable method for providing a layer of zinc on the aluminum article is to (l coat the article using the zincate process (2) strip rinse the resulting article in acid such as nitric acid and (3) electrodeposit the resulting article with more zinc in a suitable bath such as that containing about 0.5 gram zinc chloride, 0.5 gram sodium cyanide, 10.5 grams sodium hydroxide per each liter of water. Often, a current density of about 4.5 amps per square foot of article is used and the process is carried out at room temperature.

said N- Any suitable method can be used for forming a layer of copper on the aluminum or tin article.

One method of providing a layer of copper on the aluminum article is direct acid copper plating described by Atkinson, J.T.N. J. Electrochem. Soc.,-

195 8, 105 No. 27, the disclosure of which is hereby incorporated by reference. A suitable direct acid copper plating bath contains about 100 grams ammonium pyrophosphate, 60 grams oxalic acid (cryst),-20 grams copper oxide and 50 grams triethylamine, per each liter of water, the bath initially having a pH of about 5.6-5.8.

Another suitable method for forming a layer of copper on the aluminum article is to plate a layer of copper on the aluminum article that has been coated previously with a layer of zinc. Any suitable method can-be used for forming the copper layer, such as those that are ordinarily used for plating zinc with copper. One

such suitable method makes use of copper solutions which often contains the following amounts of ingredients per liter of water:

ingredient Bath Bath Bath 1 ll lll copper cyanide 40 30 l2-2l total sodium cyanide 50 '38 l7-26 sodium carbonate 30 38 12-25 free sodium cyanide 4 3-6 pH (electromctric) 10.2-10.5 9.5-l0.5

Baths I and III are operated at ll30F.' Bath ll is used at 95-l 13F. A preferred process applied a current density of 10-12 amp per square foot of workpiece, for about 2 minutes, afterwhich a current density of 10-12 amp per square foot is used.

Generally, the layer of zinc or copper that is deposited on the aluminum or tin article should be continuous and be ofa sufficient thickness to preclude substantially any gas bubbles from being formed when the aluminum or tin article having the layer of zinc or copper is coated with a film-forming polymer by electrophoretic process.

Often, the layer of zinc or copper that is deposited on the aluminum or tin article will have a thickness of at least about 0.001 mil. Frequently, the layer will have a thickness of about 0.01 to about 0. l mils. While there is theoretically no upper limit on the thickness of the layer and can be as much as about 50 mils or even greater, no benefit is seen at the present time: in using a layer having a thickness any greater than about mils.

By aluminum or tin article it is meant that the entire article can be aluminum or tin or the article can be made of some other metal or material and have an exterior coating of aluminum or tin. An example of this is tin plated steel. Aluminum or tin article means also that the article can be pure aluminum or tin or can be aluminum or tin that has been alloyed with other metals.

After the aluminum or tin article is covered with a layer of aluminum or copper, it is coated with 'a filmforming polymer by electrophoretic process. Conventional electrocoating cells can be used. Preferably, the article is coated with the film-forming polymer immediately after the aluminum article or tin article has been covered with a layer of zinc or copper or zinc and copper to thereby minimize any contamination of the surface due to dirt, grease, oxidation and the like.

Generally, aqueous polymer dispersions are used in conventional electrocoating cells. Often these poly- 4 meric dispersions have a solids content of 3-30 percent by weight and a pH of 6-1 I. A wide variety of polymers can be used such as alkyd resins, epoxy resins, acrylic resins, epoxidized drying oils, and reaction products of dicarboxylic acid anhydride and drying oil. The electrocoating compositions can contain any of the conventional pigments. Often a pigment volume concentration of up to about 30 percent is used. The pigment volume concentration is the ratio expressed on a percent basis of the volume of pigment to the total volume of pigment plus film-forming materials of the composition; the volume of pigment is the volume of vehicle displaced by the pigment wet with vehicle.

Suitable. articles include wire (round, square or rectangular), strips, rods, bars, sheets and the like.

The following examples illustrate the invention. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 The following were admixed to produce a zincate bath:

ingredient Parts H O 69.2 27.? Zinc oxide 3 1 Total: l 00 .0

Aluminum wire was annealed and degreased by passing it through a chamber containing superheated steam at about 600F. The residence time in the chamber was about .1 minute.

The annealed and degreased aluminum wire was then passed through the previously described zincate bath. The bath was at room temperature. The residence time ofthe aluminum wire in the zincate bath was about 1.33 minutes.

The aluminum wire with a layer of zinc thereon was then rinsed with water.

Following this the aluminum wire with a layer of zinc thereon was coated with film-forming polymer by electrophoretic process.

The bath in the electrocoating cell had the following composition:

Ingredient Parts acrylic polymer (acrylonitrile 63%, butylacrylate 32%,

methacrylic acid 5%), the polymer being dispersed in water, the

dispersion containing 33% polymer solids Y 13.6

water l7.6

butyrolactone 65 .0

phenolic resin (Bakelite BRL-l phenol-formaldehyde resin, 65% solids, available from Union Carbide Corp.) 3.8 Total: 1000 About a 0.4 amp current at about 225 volts was used. The residence time of the wire in the electrocoating bath was about 0.0154 minutes.

The wire with a coating of acrylic polymer thereon was then baked in a 16 foot oven having an inlet temperature of about 200F. and an outlet temperature of about 700F. The residence time in the oven was about 0.25 minutes.

Excellent resultswere obtained. The baked electrocoated acrylic polymer film appeared to be nonporous and not blistered.

EXAMPLE 2 Ingredient Parts sodium hydroxide I zinc oxide l0 polyethylene imine (PET l8" polyethylene imine having a molecular weight of about 1800, available from Dow Chemical Co.)

Aluminum wire was then coated with a layer of zinc using the above zinc electrodeposition bath. A zinc anode was used; the cathode was the aluminum wire. The current density was about 80 amps/square foot. The bath was at room temperature.

Then, the aluminum wire having a layer of zinc was rinsed with water.

Immediately after the water rinse, the wire was coated with the acrylic polymer by electrophoretic process, using the bath containing acrylic polymer described in Example I. A 200 volt current was used. The residence time was about seconds. I

The acrylic polymer coated wire was then baked. Excellent results were obtained. The baked electrocoated acrylic polymer film appeared to be nonporous and not blistered.

The invention claimed is:

1. In the process for coating a film-forming polymer on an aluminum or tin article by electrophoretic process the improvement comprising coating said article prior to said electrophoretic process with a layer of zinc, copper or zinc and copper, the layer being continuous and of a sufficient thickness to preclude substantially any gas bubbles from being formed at the aluminum or tin article when said article is coated with the film-forming polymer by the electrophoretic process.

2. The process of claim 1 wherein said layer is zinc.

3. The process of claim 1 wherein said layer is copper.

4. The process of claim 1 wherein said article is aluminum.

5. The process of claim 4 wherein said layer is copper. 0

6. The process of claim 4 wherein said layer is zinc.

7. The process of claim 6 wherein the thickness of -said layer is about 0.01-0.15 mils.

8. The process of claim 1 wherein said article is tin.

9. The process of claim 8 wherein said layer is zinc.

10. The process of claim 8 wherein said layer is copper.

11. The process of claim 1 wherein the thickness of said layer is at least 0.001 mil.

12. The process of claim 1 wherein the thickness of said layer is about 0.01- mils.

13. The process of claim 1 wherein the thickness of said layer is about 001-015 mils.

14. The process of claim 1 wherein said film-forming polymer contains pigment dispersed therein. 

2. The process of claim 1 wherein said layer is zinc.
 3. The process of claim 1 wherein said layer is copper.
 4. The process of claim 1 wherein said article is aluminum.
 5. The process of claim 4 wherein said layer is copper.
 6. The process of claim 4 wherein said layer is zinc.
 7. The process of claim 6 wherein the thickness of said layer is about 0.01-0.15 mils.
 8. The process of claim 1 wherein said article is tin.
 9. The process of claim 8 wherein said layer is zinc.
 10. The process of claim 8 wherein said layer is copper.
 11. The process of claim 1 wherein the thickness of said layer is at least 0.001 mil.
 12. The process of claim 1 wherein the thickness of said layer is about 0.01-50 mils.
 13. The process of claim 1 wherein the thickness of said layer is about 0.01-0.15 mils.
 14. The process of claim 1 wherein said film-forming polymer contains pigment dispersed therein. 